A meteorological radar makes it possible to locate precipitations such as rain, snow or hail, to measure their intensity and possibly to chart dangerous phenomena. Most meteorological radars are installed on the ground and often form part of a vaster meteorological monitoring network. But ever more airborne applications are emerging, air transport being particularly concerned by meteorological phenomena. This entails notably making it possible to sidestep cumulonimbus formations, enormous clouds that are much feared by pilots as they sometimes produce violent storms. Even airliners divert their course so as to avoid crossing the path of certain particularly threatening cumulonimbus. Specifically, lightning, hail, and strong wind shears inside the cloud add to the risk of icing and can endanger the flight if the pilot tries to pass through.
A meteorological radar allows the detection of extensive voluminal targets constituted by clouds, of which it must give the position, size and dangerousness. For this purpose, a meteorological radar may for example emit a wave in the X band. The distance to a cloud is deduced from the time necessary for the pulse emitted to perform the outward-return trip from the antenna of the radar to the cloud at the speed of light. This time corresponds simply to the duration between the emission of a pulse and the reception of its echo. Estimation of the size of a cloud involves estimating its surface area, that is to say the maximum horizontal distance over which it extends, as well as estimating its elevation, that is to say the maximum vertical distance over which it extends. Estimation of the surface area, which exploits notably the azimuthal scan of the radar beam, does not form the subject of the present invention. Estimation of the elevation, which exploits notably the elevational scan of the radar beam, forms more particularly the subject of the present invention. By way of indication, the elevation of a cumulonimbus often exceeds 10 000 metres! It is the elevation which chiefly defines the dangerousness of the cloud, since the higher a convective cloud, the more dangerous it is. But the cloud's level of dangerousness is also related to its reflectivity factor, denoted Z, which characterizes the concentration of hydrometeors in suspension in a volume of air, in liquid or solid form. In a way, the reflectivity factor Z represents the intensity of the cloud. Having passed to a logarithmic scale, it is measured in dBZ. Concretely, a display console shows the pilot a simplified representation of the clouds, with the aid of a colour code characterizing the reflectivity, whether rain, snow or hail. For example, the colour black is often used for dry air, that is to say the absence of cloud. Green and yellow can be used for medium humidity concentrations. Red is often used for zones with very strong humidity concentration, that is to say the most dangerous zones that absolutely must be sidestepped.
But constructing such a representation of a cloud is not without numerous difficulties. For example, at a given distance at which a cloud is situated, this distance being characterized by the duration between the emission of a pulse and the reception of its echo, it is difficult to estimate the size of the cloud in question. Specifically, the intensity of the echo does not make it possible to deduce the size of the cloud, since a small cloud exhibiting a high reflectivity factor can return an echo of the same intensity as a large cloud of low reflectivity. Moreover, utilizing the intensity of the echo at large distance is difficult since the echoes are weak there and mingled with the thermal noise of the radar, this being particularly troublesome.
A current solution consists in estimating pointwise, at each point of a predefined grid overlaid on the zone of interest, the intensity of the back-scattered signal. The information is derived from the radar signal relating to each of the mesh cells of the grid, and possibly adjacent mesh cells. The performance of this procedure depends first and foremost on the angular resolving power of the antenna of the radar. This is because beyond a certain distance it is no longer possible to estimate the dimensions and/or the position of a reflecting obstacle sufficiently precisely. This procedure is therefore appropriate for short-distance observations, for which the useful signal is strong and the angular resolving power of the radar high. At larger distance, the useful signal is strongly attenuated and is disturbed by the thermal noise of the radar. For certain values of the signal, it becomes impossible to determine whether the cloud lying in front of the antenna lobe is small and strongly reflective or extensive and weakly reflective, or even impossible to determine whether or not a cloud is present!
More elaborate solutions are conceivable, such as for example a solution based on estimation by minimizing an error measurement. This procedure relies on oversampling the zone to be characterized. It consists in estimating the reflectivity of a point of the grid on the basis of a series of measurements made at close points. But such a procedure would be very sensitive to thermal noise and therefore hardly effective at large distance, even though it would be desirable to allow the pilot to anticipate his sidestepping manoeuvre to the maximum. Another procedure based on deviation measurement, also known as “bi-lobing”, could for its part make it possible to tackle the problem of resolution in elevation. But a technical problem related to this procedure is to do with the resolving power of the radar antenna. Typically, the angular aperture at −3 decibels about the main lobe of a meteorological radar antenna can be of the order of 3 degrees. An angular aperture of 3 degrees corresponds to an extent of nearly 4 kilometres at 40 nautical miles and to an extent of about 10 kilometres at 100 nautical miles. This does not offer acceptable precision if it is considered that the elevation of a cumulonimbus often exceeds 10 kilometres, i.e. the same order of magnitude as the aperture of the antenna lobe: the lobe encompasses almost the entire cloud, thus intrinsically prohibiting any discrimination. The implementation of the deviation measurement based procedure is therefore not without its numerous difficulties.